1. Field of the Invention
The present invention relates to an arrangement for correcting misregistration (color misregistration) among the respective colors in an electrophotographic color image forming apparatus.
2. Description of the Related Art
Conventionally, color image forming apparatuses having independent image carriers are designed so that images of all the colors are superimposed without any deviation. However, since it is very difficult to completely eliminate variations in parts and those arising during assembly, adjusting of misregistration is necessary. One known example of such an adjustment method, as discussed in Japanese Patent Application Laid-Open No. 2002-023445, is to form a pattern for detecting misregistration, detect the position of the formed pattern, and correct a deviation amount.
In Japanese Patent Application Laid-Open No. 2002-023445, black is used as a reference color, and a pattern detection unit for detecting a pattern is configured so as to receive a specular-reflection component of light irradiated from a light emitting unit with a light receiving unit. Then, measurement color is sandwiched by the reference color, and an amount of positional deviation is calculated from a deviation amount in a result of comparison of center positions of the two reference color patterns sandwiching the measurement color and a center position of the measurement color pattern. From the calculation result, a misregistration amount among the respective colors can be corrected by adjusting image forming conditions, such as write timing of an image and an image clock.
An example of a technique for reducing a cost of a color image forming apparatus is to provide a polygonal mirror in a scanner unit to reflect light from a plurality of laser diodes for scanning. Further, a configuration which decreases a number of sensors for detecting a beam for generating a horizontal synchronizing signal is known. When the number of sensors for generating the horizontal synchronizing signal is decreased, it is known to use a pseudo-generated horizontal synchronizing signal (pseudo beam detection (BD) signal) for image formation of the colors which do not have a sensor. For example, Japanese Patent Application Laid-Open No. 2004-102276 discusses such a configuration.
In the configuration which decreases the number of sensors for beam detection for generating the horizontal synchronizing signal, how the sensors are arranged corresponding to the colors is important. In color image forming apparatuses which have a monochrome print mode (black-only printing) and a full color mode, images output in the monochrome print mode are often documents, so that quality of printed characters is important and demanded by users. An error amount of the pseudo BD signal is accumulated depending on the accuracy of calculation, sampling etc. at the time of its generation. Namely, the accumulation of error causes variation in a write position of a main scanning direction, and as a result, a problem of “jitter” is produced. Therefore, it is desirable to use a most accurately formed horizontal synchronizing signal (BD signal) for black printing synchronization. Further, it is desirable to use accurately formed black (Bk) as a reference color for registration correction.
Recently, in order to reduce costs even further, as a technique for utilizing a low-cost intermediate transfer member, a method has been discussed in which a light receiving unit receives diffuse reflected light which is independent of a surface state of the intermediate transfer member to perform registration correction.
However, when, for example as described above, black is used for the reference color to employ the sensor output for beam detection provided for black as the BD signal, and a pseudo BD signal is used for the other colors, the black pattern cannot be detected even if diffuse reflected light is employed. This is because black is a developer that shows only a small reflected component in the diffuse reflected light. If the black pattern cannot be read, the registration correction cannot be carried out.
On the other hand, recently, for color image forming apparatuses, there is a need for further miniaturization of apparatus or a need for more efficient registration correction to reduce toner consumption resulting from the registration correction.